Compact protective hoods enclose the head of a wearer in a crown either of transparent material or of opaque material with a transparent visor. Respiration is typically filtered through a mouth piece, oral-nasal cup or a full-face piece. The hood is sealed about the neck by an elastomeric dam. To make the package compact and portable the hood assembly size must be as small as possible which is accomplished by folding the hood assembly for storage until deployment.
Limitations in Fluid Impermeability Technology
Protective hoods require fluid impermeability to maintain a target protection factor. Fluid impermeability is tested by a number of methods. In one method, the hood is inflated with air and a soapy solution is applied to the exterior of the hood. Alternatively the inflated hood may be partially submerged to detect leaks. Leaks in the hood are detected by bubbles forming proximate to the leak. Leaks are most likely to occur about material interfaces such as those between the crown and the elastomeric neck dam.
As noted above, the material requirements between the crown and neck dam differ. The crown must provide a fluid impermeable three dimensional surface to surround the head of a wearer. It must interface with a visor for outward vision or be constructed of transparent material. The crown must also interface with a filtered respiratory pathway between the interior and exterior of the hood.
The neck dam must be substantially elastomeric to fit over the wearer's head and seal against the neck of the wearer. However, the neck dam must also create a fluid impermeable seal with the crown. As the crown and neck dam are typically made from different materials, this seal can be challenging to achieve effectively. Many bonding agents, tape and other methods produce an acceptable fluid impermeable seal but do not provide high mechanical strength. Stitching and other mechanical fasteners provide mechanical strength but sacrifice fluid impermeability. Both mechanical strength and fluid impermeability are inextricably intertwined as the donning of the hood introduces substantial mechanical strain on the neck dam-crown interface as the neck dam must be stretched to accommodate the greater diameter of the head of the wearer before contracting around the lesser diameter of the neck of the wearer. Additional stress is also incurred during the folding and unfolding process.